1. Field of the Invention
The present invention relates to a signal processing method, a signal processing system, a coefficient generating device, and a digital camera for correcting a noise included in a signal.
2. Description of Related Art
Generally, a FPN (Fixed Pattern Noise), vertical or horizontal streak noise, is generated on an image which is imaged with a digital camera, depending on variation in characteristics of a device that amplifies exposure-generated charge and converts it into a signal. The FPN is a noise in the same shape that is commonly generated on a plurality of images which are imaged with a same image sensor. The FPN is generated with a CCD or a CMOS image sensor. A following processing has been conventionally performed for removing an offset vertical streak FPN from an image. A calibration image is imaged with an image sensor driven in a lightproof manner. An area of a predetermined number of pixels is set for each vertical line of the calibration image. The FPN value for each vertical line is estimated from average of the pixel value in the area. An image to be corrected is imaged with the image sensor driven on exposure. Subtracting a pre-calculated FPN estimation value from the each vertical line of the image to be corrected causes the FPN to be removed from the image to be corrected. Alternatively, the FPN is estimated using OB area (an imaging area in a lightproof manner at the edge of the image sensor) as a calibration image, instead of imaging in a lightproof manner (refer to Japanese Laid Open Patent Publication No. 2003-153088).
The calculation for the FPN estimation value in conventional methods has an estimation error resulting from the influence of noise included in the calibration image other than the FPN. The sufficient number of pixels in the vertical line area for the FPN counterbalances noise other than the FPN in the vertical line area and reduces the FPN estimation error. However, the sufficient number of pixels in the vertical line area may not always be obtained due to reducing the release time lag or maintaining the speed of continuous shooting (for example about 128 pixels). In this case, the FPN estimation error is not reduced sufficiently, and a vertical streak noise is generated in a FPN-corrected image. With an conventional signal detection device, an average of the pixel value is calculated to be the FPN value in a predetermined vertical area set up in an input image. Therefore, even if the estimation value includes an estimation error, the FPN estimation value is subtracted from the input image, and such noise as the FPN estimation error may remain in a FPN-removed image.